Global change biology
Warming alters food web-driven changes in the CO2 flux of experimental pond ecosystems
T. B. Atwood1,2, E. Hammill2, P. Kratina3, H. S. Greig4, J. B. Shurin5 and J. S. Richardson1
Cite this article: Atwood TB, Hammill E, Kratina P, Greig HS, Shurin JB, Richardson JS. 2015 Warming alters food web-driven changes in the CO2 flux of experimental pond ecosystems. Biol. Lett. 11: 20150785. http://dx.doi.org/10.1098/rsbl.2015.0785
Received: 11 September 2015 Accepted: 9 November 2015
Subject Areas: ecology, environmental science Keywords: trophic cascades, biosequestration, carbon cycling, climate change
Department of Forest and Conservation Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 2 Department of Watershed Sciences and Ecology Center, Utah State University, Logan, UT 84322, USA 3 School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, UK 4 School of Biology and Ecology, University of Maine, Orono, ME 04469, USA 5 Section of Ecology, Behavior and Evolution, University of California-San Diego, La Jolla, CA 92093, USA Evidence shows the important role biota play in the carbon cycle, and strategic management of plant and animal populations could enhance CO2 uptake in aquatic ecosystems. However, it is currently unknown how managementdriven changes to community structure may interact with climate warming and other anthropogenic perturbations to alter CO2 fluxes. Here we showed that under ambient water temperatures, predators (three-spined stickleback) and nutrient enrichment synergistically increased primary producer biomass, resulting in increased CO2 uptake by mesocosms in early dawn. However, a 38C increase in water temperatures counteracted positive effects of predators and nutrients, leading to reduced primary producer biomass and a switch from CO2 influx to efflux. This confounding effect of temperature demonstrates that climate scenarios must be accounted for when undertaking ecosystem management actions to increase biosequestration.
1. Introduction Author for correspondence: T. B. Atwood e-mail: [email protected]
Electronic supplementary material is available at http://dx.doi.org/10.1098/rsbl.2015.0785 or via http://rsbl.royalsocietypublishing.org.
In freshwater ecosystems, projected increases in water temperatures (3 –58C)  are likely to interact with bottom-up and top-down processes to modify community structure  and CO2 dynamics [3,4]. Heterotrophs contribute to the net carbon balance of ecosystems by consuming organic matter and respiring it as CO2. Predators and herbivores can further influence carbon balance by directly or indirectly (via trophic cascades) shifting the balance between heterotrophic respiration and photosynthesis. However, warming and eutrophication can modify food web structure by increasing species extinctions, especially at higher trophic levels , altering species interactions. Phytoplankton blooms caused by eutrophication and trophic cascades can only enhance long-term carbon storage if plant matter escapes mineralization and is buried in sediments; however, higher water temperatures increase metabolism and remineralization rates. Because temperature modifies food web structure with consequences for CO2 assimilation and remineralization , alterations to animal and plant populations could cause complex climate feedbacks in a warmer world. We tested two hypotheses of how elevated water temperatures could alter the effects of top-down and bottom-up manipulations on the CO2 flux of freshwater pond mesocosms. Mesocosm food webs contained phytoplankton, periphyton, zooplankton, benthic macroinvertebrates and, in treatments containing fish, Gasterosteus aculeatus (three-spined stickleback; electronic supplementary material, table S1). First, warming would increase the strength of
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Table 1. Summary statistics of linear mixed-effects models for individual and interactive effects of warming (W), nutrient additions (N) and predators (P) on consumer biomass, primary producer biomass and CO2 ﬂux of mesocosms. p-values in bold are statistically signiﬁcant. primary producer biomass